Field of the Invention
The present invention relates to an image processing apparatus, method, and program for generating a two-dimensional image, which corresponds to an image acquired by simple imaging, from a tomographic image acquired by tomosynthesis imaging.
Related Art
In recent years, in radiation image capturing apparatuses using radiation such as X-rays, gamma rays, in order to observe an affected part in more detail, tomosynthesis imaging has been proposed in which a radiation source is moved to irradiate a subject from a plurality of radiation source positions to perform imaging and a tomographic image is generated by emphasizing a desired tomographic plane from a plurality of projection images acquired by the imaging. In the tomosynthesis imaging, a plurality of projection images are acquired by imaging a subject at a plurality of radiation source positions while moving a radiation source in parallel with a radiation detector or moving the radiation source so as to draw an arc of a circle or an ellipse according to the characteristics of an imaging apparatus and a required tomographic image, and these projection images are reconstructed using a back projection method, such as a simple back projection method or a filtered back projection method, to generate a tomographic image. By generating such a tomographic image on a plurality of tomographic planes of the subject, it is possible to separate structures overlapping each other in a depth direction in which the tomographic planes of the subject are aligned. Therefore, it is possible to find a lesion that has been difficult to detect in a two-dimensional image acquired by simple imaging in the related art. The term “simple imaging” refers to an imaging method for acquiring one two-dimensional image, which is a transmission image of a subject, by irradiating the subject once.
On the other hand, the tomosynthesis imaging has a problem that a reconstructed tomographic image is blurred due to the mechanical error of the imaging apparatus and the influence of body motion of the subject due to the time difference of imaging at each of a plurality of radiation source positions. When the tomographic image is blurred as described above, it is difficult to find a lesion such as minute calcification, which is useful for early detection of breast cancer, particularly in a case where the breast is a subject. For this reason, in the case of performing tomosynthesis imaging, it is common to perform simple imaging as well to acquire both a tomographic image and a two-dimensional image.
However, when both tomosynthesis imaging and simple imaging are performed, the exposure dose to the subject increases. For this reason, a method has been proposed in which a tomographic image acquired by tomosynthesis imaging is projected in a depth direction in which the tomographic planes of the subject are aligned, thereby generating a two-dimensional image corresponding to a radiation image acquired by simple imaging (refer to US2010/0135558A). Hereinafter, the two-dimensional image generated in this manner is referred to as a composite two-dimensional image.
In addition, a method has been proposed in which a projection image is acquired, frequency processing for attenuating low frequency components of a projection image, of which an angle (hereinafter, refereed to as an incidence angle) of radiation incident on the detection surface of a radiation detector with respect to an axis perpendicular to the detection surface of the radiation detector is large, relative to high-frequency components is performed, a tomographic image in which the amount of artifacts has been reduced is reconstructed from a projection image having a small incidence angle and a projection image subjected to frequency processing, and a composite two-dimensional image is generated from such a tomographic image (refer to JP2014-133095A).
In addition, a method has been proposed in which a first tomographic image that is emphasized according to the spatial frequency and that is used for image interpretation is generated based on a projection image, a second tomographic image in which the degree of emphasis is different from that of the first tomographic image and which is emphasized according to the spatial frequency, and a composite two-dimensional image is generated using the second tomographic image (refer to JP2015-66344A).
Incidentally, in the tomosynthesis imaging, the incidence angle when irradiating the subject is limited. Therefore, for example, in a case where a tomographic image is reconstructed by superimposing projection images using a back projection method, artifacts, which are virtual images of the structure, may be reflected in the depth direction in which the tomographic planes are aligned. More specifically, due to back projection, artifacts may be reflected in a region, in which no structure is present originally, of a tomographic image of a tomographic plane different from a tomographic image of a tomographic plane where a structure is present. In particular, as the size of the structure increases, a range in the depth direction in which artifacts are present increases. For this reason, in a case where a large structure and a small structure are present side by side in the depth direction, if a plurality of tomographic images are simply added up to generate a composite two-dimensional image, the small structure disappears due to the influence of artifacts of the large structure. As a result, it is difficult to check a structure required for diagnosis of a lesion or the like. In addition, the composite two-dimensional image becomes an image of an impression of being blurred.
For this reason, in the method disclosed in US2010/0135558A, a minimum value projection method is used when generating a composite two-dimensional image from the tomographic image. Here, calcification in the breast is a minute region with high brightness (that is, low density) in the projection image. Therefore, if a composite two-dimensional image is generated using the minimum value projection method, it is possible to observe a minute high-brightness region, such as calcification, in the composite two-dimensional image.